The invention relates to an apparatus and method for projecting graphic information that is divided into individual pixels onto a light-sensitive material, wherein the projected image is composed of mutually offset partial images that are represented in a transmissive device consisting of an array of transmissive elements. The apparatus includes a light source generating light rays that pass through the transmissive device in which one of the partial images is represented, and it further includes an illumination device for guiding the light rays into a plurality of beam paths extending parallel to one another through the transmissive elements, as well as a projection objective for projecting the light rays representing the partial images onto the light-sensitive material.
At the present time, the process of producing images on a light-sensitive material is performed predominantly by devices that are based on a digital operating principle. In these digitally operating devices, the graphic information to be represented in the image is divided into individual pixels that are projected onto the light-sensitive material. The individual pixels are generated by a transmissive device that is formed by a plurality of individually controllable transmissive elements, such as an LCD (liquid crystal device), a DMD (digital mirror device), or another appropriate light modulator. The transmissive elements are controlled in accordance with the graphic information that is to be represented in the image. The representation of a partial image formed by the controllable elements of the transmissive device is converted by means of an illumination device into a plurality of light rays that run along parallel beam paths and are focused by the projection objective to produce the partial image on the light-sensitive material.
The resolution of such transmissive devices depends on the number of elements, which must be addressed individually, and this number usually is not sufficient to produce a high-resolution image in photographic quality on the light-sensitive material. Therefore, as a way to increase the resolution, several partial images are produced sequentially by the transmissive device and projected onto the image plane of the light-sensitive material mutually offset to one another. For each partial image, a mask disposed in front of the transmissive device covers a section of each of the individually controllable transmissive elements, so that the partial image is formed as a raster grid of individual exposed areas that are spaced apart from one another. In order to increase the resolution, for example by a factor of four, a corresponding number of mutually offset partial images are projected onto the light-sensitive material.
A method based on the foregoing principle and a device for carrying out the method are disclosed in EP 0 987 875. According to the method described therein, the mutual offset of the partial images which are represented sequentially by the transmissive device and are projected onto the light sensitive material through an optical projection device is accomplished by means of a rotatable, inclined glass plate.
It is a disadvantage of these known devices and methods that the resolution of the image produced on the light-sensitive material depends on the number of surface segments exposed for producing each of the partial images on the light-sensitive material and, accordingly, on the size of the transmissive element sections that are not covered by the mask. If the image is to be produced on a comparatively large format of the light-sensitive material, the image resolution would be relatively coarse as a result of magnifying the image to the desired format. In other words, the raster pitch of the exposed surface elements or pixel areas of the image is magnified in the same proportion. Especially with large magnifications, the boundaries between the surface elements become fuzzy and there is overlapping of adjacent surface elements, which leads to a reduced quality and brilliance of the image. The resolution of the image can only be adapted to the format size of the light-sensitive material by exchanging the optical imaging device, the mask and the transmissive device.